Previously a furnace is known for heating a glass preform such that the glass preform may be drawn into an optical fiber. This known furnace comprises a vertical furnace core tube with a surrounding heater. The glass preform is fed into this furnace from above, and while the glass preform is heated, an optical fiber may be drawn from a softened lower end of the glass preform.
The furnace may use induction heating technique to heat up a graphite heating element, for instance, or be a resistance furnace. As graphite reacts with oxygen and with products of the dissociation of the quartz glass in high temperatures an inert gas flow is required for preventing the oxidation. The inert gas or the like, such as argon, helium or nitrogen, is supplied into the core tube.
The glass preform has an elongated round body part and a taper part on its upper end. To the upper end is connected a support rod and the support rod hangs with the glass preform. As the glass preform is fed to the furnace the changes of the glass preform diameter require a sealing arrangement to prevent the inert gases from leaking out of the drawing furnace. It is known to arrange a sleeve member to surround the outer periphery of the support rod which then closes the opening of the furnace sealing as the taper part of the glass preform passes through.
The drawback with the previous solution is a risk that the metal sleeve member surrounding the support rod enters the drawing furnace and forms a joint with the glass preform. Further, as the taper part passes through the sealing the shape of the formed flow channel for the inert gases flowing upwards out of the furnace causes turbulence to the inert gas flow and it disturbs the heating process of the preform within the drawing furnace.